Azeotropic composition

ABSTRACT

Azeotropes are formed between 1,1-dichloroethane and 1,1,2trichloro-1,2,2-trifluoroethane which boils at about 115.5* F. at 760 mm. mercury pressure. The azeotrope is useful in solvent degreasing applications.

United States Patent [56] References Cited UNlTED STATES PATENTS 2,999,817 9/1961 Bower 252/172 FOREIGN PATENTS 989,155 4/1965 Great Britain 252/172 OTHER REFERENCES Mellan-lndustrial Solvents (1950) Reinhold Pulb. Co. page 73 Rose, The Cond. Chem. Dictionary 6th Ed. Reinhold Publ. Co. pages 366, 466

Primary Examiner-Leon D. Rosdol AssistantExaminer-William E. Schulz Attorneys- Paul A. Rose, Aldo J. Cozzi and Robert C. Brown ABSTRACT: Azeotropes are formed between 1,1- dichloroethane and 1,1,2-trich1oro-l,2,2-trifluoroethane which boils at about 115.5 F. at 760 mm. mercury pressure. The azeotrope is useful in solvent degreasing applications.

AZEOTROPIC COMPOSITION This application is a continuation-in-part application of US. Pat. application Ser. No. 590,204 filed Oct. 28, 1966 and now abandoned.

This invention relates to an azeotropic composition and particularly to the azeotropic mixture of l,l,2-trichloro-l,2,2- trifluoroethane and l,ldichloroethane.

Several of the chlorofluoromethanes and chlorofluoroethanes have attained widespread use as specialty solvents in recent years, particularly trichlorofluoromethane and trichlorotrifiuoroethane. Both of these compounds are relatively low boiling liquids (CCI F, 23.8 C.;CC1,CFCC1F 47.6 C.), which are nontoxic and nonflammable, and which have satisfactory solvent power for greases, oils, waxes and the like. They have, therefore, found widespread use for cleaning electric motors, compressors, oxygen storage tanks, photographic film, lithographic plates, typewriters, instruments, gauges, sound tape, and as noncorrosive brines.

For certain solvent purposes, however, the chlorofluoromethanes and chlorofluoroethanes alone have insufficient solvent power. This is particularly true in the electronic industry during the manufacture of printed circuits. Printed circuits" are well known in the electronics art; and consist of a circuit formed from a soft metal on a solid, nonconducting surface such as a reinforced phenolic resin. During manufacture, the solid surface is coated with the metal, the desired portion of metal is coated with an impervious coating, and the excess metal is removed by etching with a suitable acid. After the excess metal has been removed, it is necessary to remove the impervious coating because solder joints must be made to the printed circuit" and these will not form if the coating is present. After the impervious coating is removed, the circuits are coated with a rosin flux to permit the joints to be soldered, which rosin flux must then be removed. The chlorofluoromethane and chlorofluoroethane solvents do not have sufficient solvent power to clean printed circuits"; that is, to effectively remove the rosin flux.

It is, therefore, an object of this invention to provide a novel azeotropic composition which has new and unusual properties. Another object is to provide an azeotropic composition which is valuable as a solvent and particularly for cleaning printed circuits. A further object is to provide an azeotropic composition which is both relatively nontoxic and nonflammable both in the liquid phase and in the vapor phase and which ,at the same time is an excellent solvent for printed circuits. It is still another object of this invention to provide a new composition of matter and to thereby advance the art.

The above object of this invention may be accomplished by the provision of a new novel azeotropic composition consisting of about 77 weight percent l,l,2trichloro-l ,2,2- trifluoroethane and 23 weight percent l,l-dichloroethane having a boiling point of l .5 F. at one atmosphere pressure.

It has been found that when l,1,2-trichloro-l,2,2- trifluoroethane and l,l-dichloroethane are admixed in the above proportions they form an azeotrope which distills at constant temperature, the liquid phase and the vapor phase in equilibrium therewith having the same composition. Such mixture is relatively nonflammable and nontoxic in both the liqiud phase and the vapor phase. These mixtures are particularly useful as solvents for greases, oils, waves, and the like and cleaning electric motors, compressors, photographic films, oxygen storage tanks, lithographic plates, typewriters, precision instruments, gauges, sound tapes, and the like and are particularly useful for cleaning printed circuits.

The azeotropic composition of this invention was obtained in the course of distillation of a mixture of 1,l,2-trichlorol,2,2-trifluoroethane and l,ldichloroethane. The mixture was charged to a laboratory distillation flask provided with an overhead condenser tor the removal of distillate. The mixture was heated to its atmospheric boiling point and distilled at high reflux. An azeotropic mixture was removed consisting of about 23 percent by weight l,ldichloroethane and about 77 percent by weight 1,1,2-trichloro-l ,2,2-trifluoroethane having a boiling o int of 1 15.5" F. at 760 millimeter ressure.

Printe circuit boards are usually prepare by impregnating glass cloth, nylon, or paper laminates with a phenol-formaldehyde resin or an epoxy resin. Printed circuits are prepared by a variety of methods. In a typical procedure, the board consists originally of a phenolic resin impregnated base to which is bonded a sheet of copper, 2 to 4 mils. thick, covering one surface of the board. The desired circuit is drawn on the copper with an asphalt based ink, using the silk screen method. The excess copper is then removed by etching with a ferric chloride-hydrochloric acid bath, sometimes containing ammonium chloride, leaving on the board the copper that is covered by the ink. After washing off the etch solution, the asphalt ink is removed by cleaning with the azeotropic composition of this invention in an ultrasonic bath (some mechanical scrubbing is often used). The entire surface of the board is coated with a rosin flux and dried. The electronics components (resistors, capacitors, etc.) are then added at the proper places for soldering to the circuit. The board is then passed over a molten solder bath, contacting the desired joints with the molten metal, whereby the soldering is effected. After cooling, the excess rosin flux remaining on the board must be removed since, if present in the final assembly, it will lead to corrosion, poor electrical resistance and other deleterious properties.

The board is cleaned by placing it in an ultrasonic bath operating at about 32 kilocycles per second and at about 25 C., where it remains for one minute.

When the board is cleaned with 1,l,2trichloro-l,2,2- trifluoroethane alone considerable amount of rosin flux remains after treatment. On the other hand, if the board is cleaned with l,l-dichloroethane alone, even though the rosin flux is substantially removed, the board itself will be considerably damaged. However, if the board is treated with the azeotropic composition of this invention substantially all of the rosin flux is removed without any detrimental effect on the board which constitutes the backing of the printed circuit.

Although the azeotropic mixture is obtained at approximately 760 mm. mercury pressure a variation in pressure and consequently a change in the composition and boiling points of the azeotrope are also obtained which is intended to be within the scope of the present invention. Thus the azeotrope may contain many different proportions of the aforementioned components provided a constant boiling mixture is obtained at the various fixed pressures at which the compositions are used. Stated otherwise, any pressure may be used to obtain the azeotropes of the present invention as long as a two-component constant boiling mixture is obtained at any one fixed pressure within a range. Accordingly the ratio of components of the azeotropes will also vary over a range of pressures (e.g. fluctuations in ambient pressure); but the ratio of components will remain fixed when the pressure is fixed. The azeotropes of the invention, therefore, comprise those which vary about :10 percent and especially those that vary about fi percent of the quantity of components obtained for a constant boiling azeotropic mixture at 760 mm. mercury pressure. Therefore, the azeotropes of the present invention comprise from about 25.3 to about 20.7 and especially from about 24.15 to about 21.85 parts by weight of l,l-dichloroethane and from about 84.7 to about 69.3 especially from about 80.85 to about 73.15 parts by weight of 1,1 ,Z-trichloro-1,2,2-trifluoroethane.

What is claimed is:

1. A composition of matter consisting essentially of an azeotrope which at about 760 millimeters of mercury pressure and at about 115.5 F. consists essentially of about 23 percent by weight l,l-dichloroethane and about 77 percent by weight of l, l ,2-trichlorol ,2,2-trifluoroethane.

2. A method for cleaning a surface by contacting said surface with a composition of matter consisting essentially of an azeotrope which at about 760 millimeters of mercury pressure and at about 1 F. consists essentially of about 23 percent by weight l,l-dichloroethane and about 77 percent by weight of l l ,2-trichlorol ,2,2 trifluoroethane. 

2. A method for cleaning a surface by contacting said surface with a composition of matter consisting essentially of an azeotrope which at about 760 millimeters of mercury pressure and at about 115.5* F. consists essentially of about 23 percent by weight 1,1-dichloroethane and about 77 percent by weight of 1,1, 2-trichloro-1,2,2-trifluoroethane. 